Myxomatous mitral valve (MV) disease (MMVD) affects over 140 million patients worldwide. This application is concerned with maintaining and restoring MV homeostasis in MMVD patients through investigations of serotonin (5HT) mechanisms that affect MMVD pathophysiology. 5HT is a neurotransmitter, which if present at increased levels can cause valvulopathies. Our research indicates that progression of MMVD is mediated by 5HT signaling via 5HT receptors (5HTR) 2A and 2B and by a specific 5HT transporter (SERT) polymorphism in the promoter region (LL-SERT, the full-length promoter) with a Mendelian distribution. Thus, MMVD appears to be a disorder with increased 5HTR responsiveness due to augmented levels of 5HTRs, and is exacerbated by maximal expression of SERT due to the LL-SERT genotype. Downstream processing post-SERT of 5HT results in increased reactive oxygen species (ROS) and TGF-?1 signaling. Specific Aim 1: To determine the impact of LL-SERT polymorphism in MMVD progression and recurrence after surgical repair and 5HT responsiveness. 1. a) Hypothesis: Patients with LL-SERT polymorphism are at higher risk of more rapid progression of MMVD, requiring surgery at a younger age; furthermore post-surgery this genotype with MMVD may have an increased risk for re-operation due to continuing 5HT-enhanced mechanisms. We will conduct genetic and bioinformatics analyses to determine occurrence, progression rate, and clinical recurrence of MVD patients based on SERT polymorphisms, SERT SNP variations, and imaging studies. 1. b) Hypothesis: VIC derived from patients with LL-SERT polymorphism are more responsive to 5HT stimulation resulting in MVICs activation and tissue remodeling. LL-, LS-, and SS-derived VICs and MV tissues will be isolated from MMVD patients and controls and exposed to 5HT stimulation. Endpoints will be MVICs phenotype, ROS production, and ECM remodeling. Specific Aim 2: To investigate the mechanisms of 5HTR signaling in human derived MV cells from all 3 SERT genotypes, focusing on TGF-?1 related pathways. 2. a) Hypothesis: Suppression of 5HTR2A & 2B expression in human derived MV cells and tissues controls cell phenotype and ECM. This subaim will use established siRNA constructs specific for 5HTR2A/2B to investigate their effects over a 5HT dose range, with endpoints including cell proliferation and ECM remodeling. This approach will also study SERT genotypes, SS, LL, and LS separately. 2. b) Hypothesis: Downstream TGF-?1 events related to 5HTR signaling mediate the progression of MMVD. These studies will investigate the effect of 5HTR-TGF-?1 interactions including studies of TGF- ?1-receptor expression, TGF-?1 excretion, canonical and non-canonical signaling and the effects of TGF-?1 arising from 5HTR activity on ROS and ECM.